Industrially developed countries generate billions of tons of air pollutants, a great part accounts to the combustion of coal, oil, and gasoline in electric power plants. Other major air pollution sources include petroleum refineries, cement plants and petrochemical plants.
Among the most troublesome air pollutants emitted into the atmosphere are acid gases and in particular SO2 and NOx. These gases are the major cause for acid rain, for smog and for human discomfort and disability. Sulfur dioxide is released into the atmosphere from the combustion of sulfur containing compounds in fossil fuels, such as gas, petroleum, and coal. SO2, is a heavy, colorless gas with a characteristic, suffocating odor. In moist air it is slowly oxidized to sulfuric acid and contributes to acid rain. NOx gases, on the other hand, partly originate from the combustion of nitrogen containing compounds but mostly they originate from the reaction between elementary oxygen and nitrogen present in the air at the elevated temperatures at which various industrial processes (and combustion) take place. Therefore, the problem of NOx is even more difficult to overcome as it is a problem inherent to these processes and its generation cannot be avoided. Nitrogen oxides, in addition to their contribution to acid rain, breaks down to form ozone and reacts with other atmospheric pollutants to form photochemical smog which irritates sensitive membranes and damages plants.
The increasing environmental awareness during the last decades has led in many countries to governmental regulations, enforcing standards for maximum air pollutants emission on power plants and industries, in order to achieve air quality standards for various hazard materials. For example, in the U.S., the Clean Air Act of 1967 as amended in 1970, 1977, and 1990 is the legal basis for air-pollution control throughout the U.S. The 1990 amendments to the Clean Air Act of 1967 put in place regulations to reduce the release of sulfur dioxide from power plants to 10 million tons per year by Jan. 1, 2000. This amount is about one-half the emissions of 1990. In 1988, as part of the United Nations-sponsored Long-Range Transboundary Air Pollution Agreement, the United States, along with 24 other nations, ratified a protocol freezing the rate of nitrogen oxides emissions at 1987 levels.
The need to obey these regulations has led to the development of diversified methods for controlling and reducing the emission of air pollutants. In general, these methods include removing the hazardous material before it is used (for example, using low-content sulfur coal) or removing the pollutant after it is formed. For some air pollutants, (e.g., NOx) only methods of the second class can be employed.
The present invention relates to methods of the second class.
Industrially emitted pollutant gases can be entrapped by liquids or solids traps that adsorb the harmful gases before they are released into the atmosphere. These traps are usually in the form of tower tanks contactors through which the waste gas is passed upward while the liquid, or a slurry, i.e. a mixture of liquids and solids, is descending downward. The agent which absorbs the pollutants and prevent their emitting into the atmosphere is called the scrubbing agent. When this agent is a liquid, it is sometimes the practice to fill the tower with inert particles in order to increase the contact surface between the scrubbing agent and the waste gas and to increase the residence time of the gas inside the reactor. In such cases it is also the practice to circulate the liquid scrubbing agent through the reactor until it is loaded with the pollutants.
Wet flue gas desulfurization, i.e., the removal of SO2 processes typically involve the use of an alkaline cleansing liquid, such as a calcium-based slurry or a sodium-based or ammonia-based solution. These processes however are not suitable for the removal of nitrogen oxides. The currently used methods for the removal of NOx from flue gas are mainly of the king that is based on the reaction of NO and NO2 with ammonia, with or without a catalyst, forming nitrogen and water. These methods are effective only within a narrow flue gas temperature range, are relatively high cost and involve the risk of ammonia leakage to the atmosphere. In the presence of catalyst the process is more efficient but its cost is even higher and the catalyst tends to be poisoned. In any case these methods require a special equipment useful only for the removal of NOx.
U.S. Pat. No. 3,6707,004 discloses a process for removing traces of hydrogen sulfide (H2S) contained in gases. The process consists of passing the hydrogen sulfide-containing gas through a liquid phase consisting essentially of iodine dissolved in an organic solvent.
U.S. Pat. No. 3,784,478 teaches a process for removal of nitrogen oxides from the gaseous effluents of combustion processes utilizing gas-liquid absorption.
Russian patent No. 2099789 describes a process for removing sulphur dioxide from gasses by absorption with sulphoxides.
The processes described in U.S. Pat. Nos. 3,6707,004, 3,784,478 and Russian patent No. 2099789 are disadvantageous since by these processes only one gas (NOx or SO2 or H2O) can be removed from waste gases.
Therefore a great effort is put into developing new NOx control methods which will remove simultaneously NO2 and SO2 from combustion flue gas. For example, U.S. Pat. No. 4,418,044 teaches a scrubbing agent consisting of a solution of Fe(II) ions and thiosulfate in a miscible mixture of water and alcohol. U.S. Pat. No.4,885,146 teaches a similar scrubbing agent consisting of Fe(II) ions but in non-aqueous solvent (only up to 10% water). According to U.S. Pat. No. 4,885,146 the scrubbing agent can be regenerated for reuse.
The cost of pollution reduction processes and equipment is, however, still very high and therefore intensive efforts are put into research in order to increase the efficiency of known processes and in order to find new more efficient and less energy consuming methods. These efforts concentrate on the following points: simplicity of process (i.e., involve low-cost equipment), the use of low-cost materials and the recycling of these materials, universality of process, i.e., removing as many pollutants as possible by the same process. Other, no less important, efforts are dedicated to finding processes in which the end products, not only are harmful, but can also be collected and be sold as useful materials (i.e., converting the harmful air pollutants into valuable materials) so that at least part of the investment will be earned back.
Therefore there is still a need for developing simple, more efficient and relatively low-cost, methods for air pollution control, especially methods which will clean fuel gases from both SO2 and NOx simultaneously in one process and with the same equipment.
It is the aim of the present invention to provide a method for removing acid gases, in particular SO2 and NOx, simultaneously.
Yet it is another aim of the present invention to provide such a method which involves low-cost materials and equipment.
It is yet another aim of the present invention to provide such a method in which the scrubbing material is regenerated easily for further use.
It is another aim of the present invention to provide such a method which will also results in conversing the SO2 and NOx into useful materials with commercial value.